Humidity-Independent Gas Sensors Using Pr-Doped In₂O₃ Macroporous Spheres: Role of Cyclic Pr³⁺/Pr⁴⁺ Redox Reactions in Suppression of Water-Poisoning Effect

Abstract

Pure and 3-12 at. % Pr-doped In₂O₃ macroporous spheres were fabricated by ultrasonic spray pyrolysis and their acetone-sensing characteristics under dry and humid conditions were investigated to design humidity-independent gas sensors. The 12 at. % Pr-doped In₂O₃ sensor exhibited approximately the same acetone responses and sensor resistances at 450 °C regardless of the humidity variation, whereas the pure In₂O₃ exhibited significant deterioration in gas-sensing characteristics upon the change in the atmosphere, from dry to humid (relative humidity: 80%). Moreover, the 12 at. % Pr-doped In₂O₃ sensor exhibited a high response to acetone with negligible cross responses to interfering gases (NH₃, CO, benzene, toluene, NO₂, and H₂) under the highly humid atmosphere. The mechanism for the humidity-immune gas-sensing characteristics was investigated by X-ray photoelectron and diffuse reflectance infrared Fourier transform spectroscopies together with the phenomenological gas-sensing results and discussed in relation with Pr³⁺/Pr⁴⁺ redox pairs, regenerative oxygen adsorption, and scavenging of hydroxyl groups.